A gas sensor is described, for example, in German Published Patent Application No. 198 17 012. The gas sensor includes a sensor element which is fastened in a housing and is sealed. The sensor element is built up in layers by planar technique, and has a first and a second solid electrolyte layers. Between the two solid electrolyte layers a measuring gas chamber is provided, in which a first and a second annular electrodes are arranged on opposite sides. The measuring gas present outside the sensor elements, whose oxygen concentration is to be determined using the gas sensor, is able to reach the measuring gas chamber via a gas access opening in the first solid electrolyte layer and via a hollow cylinder-shaped diffusion barrier. The diffusion barrier is arranged in the layer plane between the first and the second solid electrolyte layers. The inner and outer lateral surfaces of the diffusion barrier are aligned perpendicular to the layer plane of the sensor element. The gas access opening is produced with the aid of a bore that is carried out right into the diffusion barrier. Consequently, the gas access opening and the diffusion barrier have the same internal diameter. The measuring gas chamber is sealed laterally by a sealing frame. Such planar sensor elements may be produced by printing functional layers onto ceramic blank foils (unsintered solid electrolyte layers) using silk-screen printing. The printed blank foils are laminated together and sintered. The first solid electrolyte layer is formed before sintering by a first blank foil, and the second electrolyte layer is formed before sintering by a second blank foil.
In order to produce the structure between the first and the second solid electrolyte layers, for example, the annular first electrolyte is first applied onto the first blank foil, followed by the hollow cylinder-shaped diffusion barrier. Subsequently, over the first electrode, as well as in the opening of the diffusion barrier, a cavity paste containing a pore-forming material is applied, which, after sintering, forms the measuring gas chamber, or rather a section of the gas access opening. Finally, the second electrode is applied over the cavity paste for the measuring gas chamber. The printed first blank foil (print substrate) is laminated together with the non-printed second blank foil.
Because of the silk-screen printing process and on account of the properties of the paste forming the diffusion barrier, the lateral surface of the diffusion barrier, after sintering, is aligned inclined towards the large surface of the sensor element. Consequently, the diffusion barrier has a smaller inside diameter on the side facing the print substrate (first solid electrolyte layer), and a larger outside diameter than on the side facing away from the print substrate. With that, the path the exhaust gas has to travel in the diffusion barrier is greater on the side facing the first solid electrolyte than on the side facing the second solid electrolyte. In oxygen probes that work on the limiting current principle, since the measuring result is a function of the diffusion of the measuring gas through the diffusion barrier, it is desirable for the diffusion barrier to be constructed as symmetrically as possible, so that the path covered by the exhaust gas within the diffusion barrier varies as little as possible.
In diffusion barriers having slanted lateral surfaces, the dependence of the measuring signal on dynamic pressure changes increases. This is undesirable, since, for the engine control, the oxygen concentration in the exhaust gas, and not the partial pressure of the oxygen, is required.
The diffusion barrier may become detached from the second solid electrolyte layer in response to a strong shaking load, since the surface over which the diffusion barrier is connected to the second solid electrolyte is smaller than the surface over which the diffusion barrier is connected to the first solid electrolyte layer (the print substrate).
If the opening in the diffusion barrier is produced with the aid of a bore, so that the opening is aligned perpendicular to the large surface of the sensor element, then the cost of manufacturing the bore becomes expensive, and the inside diameter of the diffusion barrier may not be able to be designed to be greater than the diameter of the gas access opening.